The present invention relates to a photoelectric intrusion detector which detects interruption of a beam of optical radiation, e.g., infrared radiation, by an intruder to generate an alarm output.
Systems for detecting interruption of a beam of infrared radiation to give an alarm indicating the presence of an intruder are known in the art as shown in U.S. Pat. No. 3,752,978 to W. G. Kahl, Jr, et al or U.S. Pat. No. 4,516,115 to R. A. Frigon et al.
However, the conventional intrusion detectors have been disadvantageous in that while the outdoor use of the detector in a clear air condition such as fine weather does not greatly attenuate the arriving pulsed light from a light emitting unit during its propagation through space, thus ensuring a received light level of a sufficient light intensity at a light receiving unit, the occurrence of fog, rainfall or the like increases the attenuation of the arriving pulsed light during its propagation through space so that the received light level is decreased and the resulting input voltage to a level comparator within the receiving-end unit becomes lower than a predetermined reference voltage, thereby giving rise to the danger of issuing a false alarm.
Although the occurrence of such a false alarm due to fog or rainfall in cloudy weather can be prevented by increasing the intensity of the pulsed light from the light emitting unit, there is of course a limitation to the light emission power of an infrared light emitting diode of the type generally used in the light emitting unit. Therefore, if the established warning distance between the transmitting and receiving units is increased, the attenuation of the pulsed light reaches to a level which cannot be ignored, thereby tending to cause a false alarm in cloudy weather and thus there is a restriction that the established warning distance cannot be increased considerably.